2. Table of Contents

3. Introduction

NTP supports a large number of satellite, radio and telephone modem
reference clocks, plus a special pseudo-clock used for backup or when no other
clock source is available.

A general description of the reference clock support is on this
page. Additional information about each reference clock driver can be
found via links from here. Additional information is on the
Debugging Hints for Reference Clock Drivers and
How To Write a Reference Clock Driver pages.
Information on how to support pulse-per-second (PPS) signals produced by
some devices is on the Pulse-per-second (PPS) Signal
Interfacing page. All reference clock drivers require that the
reference clock use only Coordinated Universal Time (UTC). Timezone and
standard/daylight adjustments are performed by the operating system
kernel.

Nowadays a reference clock will generally (though not always) be a GPS
or GPSDO (GPS-disciplined oscillator). In former times it was often a
radio timecode receiver synchronized to standard time as provided by
NIST and USNO in the US, NRC in Canada and their counterparts
elsewhere in the world. Precision time radios have been extinct in
the U.S. since NIST changed their signal modulation at
2012-10-29T15:00:00Z; elsewhere they still exist but usage is
declining under pressure from GPS technology.

A device driver specific to each reference clock must be compiled in
the distribution; however, most common GPS, radio, satellite and
telephone modem clocks are included by default and are activated by
configuration commands. Note that an attempt to configure a reference
clock when the driver has not been compiled or the hardware port has
not been appropriately configured results in a scalding remark to the
system log file, but is otherwise non hazardous.

Reference clocks are supported in the same way as ordinary NTP servers
and use the same filter, select, cluster and combine algorithms. The
connection to the computer is device dependent - usually a serial
port. The particular device is specified by adding a soft link from
the name used by the driver to the particular device name.

The refclock command is used to configure a reference clock. The
options resemble those of the server directives, but mode,
minpoll, maxpoll, and prefer options are supported for reference
clocks, as described on the Reference Clock
Commands page. The prefer option can be useful to persuade the
server to cherish a reference clock with somewhat more enthusiasm than
other reference clocks or peers. It is further discussed on the
Mitigation Rules and the prefer Keyword page. The
minpoll and maxpoll options have meaning only for selected clock
drivers.

Additionally, the refid and stratum options can be used to
override the defaults for the device. There are two optional
device-dependent time offsets and four flags that can be included in
the refclock command as well.

The stratum number of a reference clock is by default zero. Since the
ntpd(8) daemon adds one to the stratum of each peer, a primary
server ordinarily displays an external stratum of one. In order to
provide engineered backups, it is often useful to specify the
reference clock stratum as greater than zero. The stratum option is
used for this purpose. Also, in cases involving both a reference clock
and a pulse-per-second (PPS) discipline signal, it may be useful to
specify the reference clock identifier as other than the default,
depending on the driver. The refid option is used for this
purpose. Except where noted, these options apply to all clock drivers.

4. Special Considerations

The Undisciplined Local Clock driver can
simulate a reference clock when no external synchronization sources are
available. If a server with this driver is connected directly or
indirectly to the public Internet, there is some danger that it can
destabilize other clients. It is not recommended that the local clock
driver be used in this way, as the orphan mode described on the
Association Management page provides a generic backup
capability.

The local clock driver can also be used when an external synchronization
source such as the IEEE 1588 Precision Time Protocol or NIST Lockclock
directly synchronizes the computer time. Further information is on the
External Clock Discipline and the Local Clock Driver
page.

Several drivers make use of the pulse-per-second (PPS) signal
discipline, which is part of the generic driver interface, so require
no specific configuration. For those drivers that do not use this
interface, the PPS Clock Discipline driver can
provide this function. It normally works in conjunction with the
reference clock that produces the timecode signal, but can work with
another driver or remote server. When PPS kernel features are present,
the driver can redirect the PPS signal to the kernel.

Some drivers depending on longwave or shortwave radio services need to
know the radio propagation time from the transmitter to the receiver.
This must be calculated for each specific receiver location and requires
the geographic coordinates of both the transmitter and receiver. The
transmitter coordinates for various radio services are given in the
Time and Frequency
Standard Station Information page. Receiver coordinates can be obtained
locally or from Google Earth. The actual calculations are beyond the
scope of this document.

Depending on interface type, port speed, etc., a reference clock can
have a small residual offset relative to another. To reduce the effects
of jitter when switching from one driver to the another, it is useful to
calibrate the drivers to a common ensemble offset. The
enable calibrate configuration command described on the
Miscellaneous Options page activates a special
feature which automatically calculates a correction factor for each
driver relative to an association designated the prefer peer.

5. List of Reference Clock Drivers

Following is a list showing the type name and title of each driver
currently implemented. Click on a selected type for specific
description and configuration documentation.

If you have seen older versions of NTP, this list may have fewer
entries than you expected. Support for some very ancient drivers
(notably, those rendered obsolete by the WWVB modulation change at
2012-10-29T15:00:00Z) has been dropped in order to reduce our
maintenance load. So have some other drivers (notably the Austron
2200A/2201A and Magnavox MX4200) after having been end-of-lifed with
no sign of aftermarket activity for more than ten years. Several
others have been removed for relying on obsolete buses or hardware
classes that no longer exist.

For security reasons, we will no longer support any refclock that
requires a closed-source driver to run. This filtered out the
Datum/Bancomm/Symmetricom bc600-series GPS/IRIG Receiver, the Hopf
GPS/DCF77 6039 for PCI-Bus, and the Spectracom TSYNC PCI. The Hopf
6021 driver has also been removed because it duplicates support for
the 6021 in the generic parse driver.